The disclosures herein relate generally to electronic packages and more particularly to a cooling apparatus for electronic packages.
Many electronic packages include electronic devices such as central processing units (commonly referred to as the microprocessor) that generate a considerable amount of heat. The performance and operating life of these devices are adversely affected by excessive temperatures. As a result, it is necessary to use a cooling apparatus to control the operating temperature of these types of heat generating devices.
U.S. Pat. No. 5,345,107 discloses a cooling apparatus for an electronic device. A cooling solid body is in close contact through a thermally conductive fluid with a heat transfer portion of the electronic device. A number of grooves communicate with the outside of the heat transfer portion of the electronic device. A spring member forces the cooling solid body into close contact with the electronic device to enhance heat transfer.
U.S. Pat. No. 5,184,211 discloses a packaging and cooling assembly for integrated circuit chips. The assembly includes a base for receiving one or more circuit chips and a combination heat sink and cover for attachment to the base. Compliant cushions that generally conform to the shape and size of the chips are held loosely between the circuit sides of the chips and the base. The heat sink engages the back sides of the circuit chips when it is attached to the base. This causes the chips to compress the compliant cushions, thereby holding the chips firmly in position, and forming a high thermal conductivity interface between the chips and the heat sink. To further enhance the heat transfer characteristics of the interface, a thin film of fluid is coated on the back sides of each chip to fill in the microvoids which result from uneven contact of the heat sink and chip mating surfaces. A sealing gasket is provided between the heat sink and the base to form a protective enclosure for the chips.
U.S. Pat. No. 4,414,562 discloses a semiconductor assembly, comprising a flat disc-shaped semiconductor with terminals on opposite faces, with an electrically conductive heat sink in electrical and thermal contact with each face. The semiconductor device and heat sinks are enclosed and maintained under compression by a rigid frame. The heat sinks are electrically insulated from each other and from the rigid frame by an insulating bond which secures these components together. Variations in compressive pressure, holding the heat sinks in facing contact with the semiconductor device, is compensated by a thermally responsive element positioned between each heat sink and the rigid frame in linear alignment with each other and the semiconductor device. Increases in temperature will expand the thermally responsive element to increase compression forces on the assembly of heat sinks and semiconductor devices
U.S. Pat. No. 4,233,645 discloses a semiconductor device package having a substrate, one or more semiconductor devices mounted on a top surface of the substrate, and a heat sink having a surface in opposed spaced parallel relation to the top surface of the substrate. At least one deformable heat transfer member is positioned between a device mounted on the top surface of the substrate and the surface of the heat sink. The heat transfer member includes a porous block of material having a heat conductive non-volatile liquid retained within the block of material by a surface tension. The heat transfer member is operative to transfer heat from the device to the heat sink.
In a portable computer, the use of a cooling apparatus to control the operating temperature of the microprocessor is essential. Furthermore, maintaining a suitable thermal interface between the microprocessor and the attached cooling apparatus is extremely critical. If the microprocessor temperature is not kept within the specification limits of the component, the effective life and performance of the microprocessor will be greatly diminished. This is especially true as the power consumption of semiconductor devices such as microprocessors continues to increase. Minimizing the thermal resistance between the microprocessor and the attached cooling apparatus can greatly improve cooling efficiency. This will often translate to less weight and/or higher performance of the cooling apparatus.
Direct contact of a cooling apparatus such as a heat sink or a thermal block to the microprocessor is undesirable in many applications. Recently, some manufacturers have begun designing their microprocessors without an integral heat plate. As a result, the microprocessor can be easily damaged by stresses caused by direct contact with an object such as a heatsink or thermal block.
To establish effective thermal transfer between two mechanically fastened parts, a suitable thermal interface material is needed to ensure maximum surface-to-surface contact. Mainly, three types of industry standard thermal interface materials are used: thermal pads, phase change materials and thermal grease. Thermal greases and phase change materials can have less than one-tenth the thermal resistance of thermal pads. The superior thermal resistance is partially because the thermal grease and the phase change material are "flowable" materials. They flow to conform to surface imperfections in the semiconductor and in the contact surface of the cooling device. Their effectiveness is also not significantly impacted by misalignment between the heat generating device and the cooling apparatus.
Although flowable materials work well at initial application, the effectiveness of the thermal interface will degrade over time due to a natural phenomenon often referred to as the "pumping effect". The pumping effect happens when the thermal interface material experiences heating and cooling cycles. The interface material expands when it is heated. The expanded material flows out of the interface area. Upon returning to room temperature, the material contracts and flows back to its original volume.
However, after repeated thermal cycling, not all of the material flows back into the interface area. Due to the continuous loss of thermal interface material over time, the interface will eventually be devoid of enough of the flowable material to effectively conduct heat away from the heat generating device. The resulting interface exhibits a much higher thermal resistance than would have been achieved with direct surface-to-surface contact.
Due to the pumping effect, the better performing thermal interface materials such as thermal grease or phase change materials are typically not used in mass produced electronic equipment such as computers. The maximum efficiency of the cooling apparatus is limited and the long term performance is unpredictable. As a result, for long term use in mass produced computer systems, only thermal pads are used to effectively interface the microprocessor and the attached cooling apparatus.
Accordingly, there is a need for a cooling apparatus that allows flowable thermal interface materials to be used in the mass production of electronic devices such as computers.